The present invention relates in general to reproduction apparatus utilizing an electrically biased roller for transferring a marking particle image from an image bearing dielectric support member to a receiver member, and more particularly to control for the electrical bias of the reproduction apparatus transfer roller in order to optimize cleaning thereof.
In typical commercial electrostatographic reproduction apparatus (copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged charge-retentive or photo-conductive member having dielectric characteristics (hereinafter referred to as the dielectric support member). Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the dielectric support member. A receiver member, such as a sheet of paper, transparency or other medium, is then brought into contact with the dielectric support member, and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member, and the image is fixed (fused) to the receiver member by heat and pressure to form a permanent reproduction thereon.
Application of the electric field to effect marking particle image transfer is generally accomplished by ion emission from a corona charger onto the receiver member while in contact with the dielectric support member. Alternatively, an electrically biased roller, urging the receiver member against the dielectric support member, has been used to cause the marking particles on the dielectric support member to move to the receiver members. That is, the transfer roller is electrically biased so as to charge the receiver member with the opposite polarity to that of the marking particles. Roller transfer apparatus offer certain advantages over corona transfer apparatus in that the roller transfer apparatus substantially eliminate defects in the transferred image due to paper cockle or marking particle flakes. This result stems from the fact that the pressure of the roller urging the receiver member against the dielectric support member is remarkably efficient in providing intimate uniform contact therebetween.
However, during operation of roller transfer apparatus, background marking particles, or marking particles outside the area of the receiver member may be picked up by the transfer roller resulting in contamination of the roller. Transfer roller contamination may eventually result in contamination of the backside of receiver members passing between the transfer roller and the dielectric support member. The backside of the receiver members are those sides facing the transfer roller surface. In order to minimize transfer roller contamination, a cleaning subsystem may be added to the roller transfer assembly. The cleaning subsystem which is preferentially used in current practice is that of a rotating fur brush and an associated vacuum. The fur brush typically rotates at high speeds, and the vacuum induced high air velocity is required to clean the brush and transport the airborne marking particles and other contaminants to a filter. Thus it can be appreciated that roller transfer apparatus are more complex than corona transfer apparatus in that they require cleaning due to their tendency to pick up marking particles from the dielectric support member and undesirably deposit such particles on the back side of the receiver member. Further, the roller transfer apparatus, including their respective cleaning assemblies, must be constructed so as not to interfere with ready clearance of any jammed receiver members.
Examples of selectively positionable roller transfer apparatus constructed to include integral cleaning mechanisms, are shown in U.S. Pat. Nos. 5,101,238 (issued Mar. 31, 1992, in the names of Creveling et al), and 5,491,544 (issued Feb. 13, 1996, in the names of Kenin et al). While roller transfer apparatus with associated cleaning mechanisms of this type are generally effective in providing for reliable image transfer to receiver members and efficient transfer roller cleaning, under certain circumstances the transfer roller cleaning is insufficient. This is particularly the case with charged area development (CAD) or discharged area development (DAD) where process control patches are developed in the interframe between marking particle images. Contamination is also picked up by the transfer roller from the dielectric support member splice. The cleaning mechanisms described in the aforementioned patents can be ineffective as presently configured to handle such process control patch contamination or dielectric support member splice contamination picked up by the transfer roller. Further, in discharge area development (DAD), the contamination problem may be accentuated (may be material dependent). This is due to the polarity of charge on residual marking particles, or marking particles in the interframe between images, urging the marking particles to the transfer roller to contaminate the roller.